polymers Article
Effect of Preparation Methods on the Tensile, Morphology and Solar Energy Conversion Efficiency of RGO/PMMA Nanocomposites Shin Yiing Kee 1 , Yamuna Munusamy 1, *, Kok Seng Ong 2 and Koon Chun Lai 1 1
2
*
Department of PetroChemical Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Kampar, Perak 31900, Malaysia;
[email protected] (S.Y.K.);
[email protected] (K.C.L.) Department of Industrial Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Kampar, Perak 31900, Malaysia;
[email protected] Correspondence:
[email protected]; Tel.: +60-54688888
Academic Editor: Wei Min Huang Received: 1 May 2017; Accepted: 14 June 2017; Published: 18 June 2017
Abstract: In this study, reduced graphene oxide (RGO)/polymethyl methacrylate (PMMA) nanocomposites were prepared by employing in situ polymerization and solution blending methods. In terms of mechanical properties, RGO loading increased the Young’s modulus but decreased the elongation at break for RGO/PMMA nanocomposites. Tensile strength for solution blended RGO/PMMA nanocomposites increased after adding 0.5 wt % RGO, which was attributed to the good dispersion of RGO in the nanocomposites as evidenced from SEM and TEM. Solar energy conversion efficiency measurement results showed that the optimum concentration of RGO in the RGO/PMMA nanocomposites was found to be 1.0 wt % in order to achieve the maximum solar energy conversion efficiency of 25%. In the present study, the solution blended nanocomposites exhibited better overall properties than in situ polymerized nanocomposites owing to the better dispersion of RGO in solution blending. These findings would contribute to future work in search of higher conversion efficiency using nanocomposites. Keywords: RGO; PMMA; solar energy conversion efficiency; morphology; synthesis method
1. Introduction Graphene is a monolayer hexagonal sp2 hybridized carbon sheet. It has received much attention in recent years due to its high mechanical strength, large specific surface area, and excellent thermal and electrical conductivity [1,2]. Graphene can be synthesized through various methods, such as chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), graphitization of a carbon-containing substrate, solvothermal, organic synthesis, and chemical reduction of graphene oxide [3–5]. Graphene obtained by the reduction of graphene oxide through thermal, chemical or electrical treatments is generally known as reduced graphene oxide (RGO). Research works involving graphene/polymer nanocomposites have been carried out widely with regard to many applications, such as electromagnetic shielding, corrosion-resistant coating, antistatic, lithium ion batteries, supercapacitors, fuel cells, photovoltaic devices, biosensing systems and photocatalysts [6–8]. Researchers have synthesized graphene/polymer nanocomposites with several polymer matrices such as polystyrene, poly(vinyl alcohol), low density polyethylene, polymethyl methacrylate, epoxy and natural rubber composite using different preparation methods like melt intercalation, solution blending, in situ polymerization, etc. [9–17]. Despite various materials having been proposed for heat dissipation [18,19], the highly thermally conductive graphene-based nanocomposites have recently
Polymers 2017, 9, 230; doi:10.3390/polym9060230
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been the subject of focus in thermal management applications. Previous works showed that graphene can significantly improve the electrical, thermal and mechanical properties of graphene/polymer composites. Pham et al. [20] reported that the electrical conductivity of PMMA-RGO composites prepared by a simple latex emulsion polymerization method was 64.1 S/m at 2.7 vol % of RGO. Additionally, the thermal conductivity of epoxy and a mixture of graphene and multilayer graphene at 10 vol % loading was found to be about 2400% higher than the thermal conductivity of pristine epoxy [21]. Graphene is efficient for energy harvesting, which is attributed to its large optical absorptivity and charge mobilities. Optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and interband transitions at higher energies (from mid-infrared to ultraviolet) [22]. During the interband transitions, the phonons (lattice vibration quanta) are exchanged with the lattice, and the reactions lead to a high thermal capacity. The absorption peak for PMMA and RGO is at about 190–260 nm and 268 nm, respectively [23,24]. The solar energy conversion efficiency of graphene/polymer nanocomposites is important for extending the application of graphene into thermal energy storage [25–27]. Though many research works have been carried out on the measurement of thermal conductivity and electrical conductivity of graphene/polymer nanocomposites, solar energy conversion efficiency of the graphene/polymer nanocomposites has nevertheless not gained much attention. Thus, the effect of PMMA/RGO preparation methods on the tensile strength, morphological properties and solar energy conversion efficiency of the nanocomposites should be studied in detail. PMMA is selected in this study, owing to its optically clear amorphous thermoplastic behavior, with high light transmittance and its resistance to chemical and weathering effects [28]. As the tensile properties of the RGO/PMMA nanocomposites depend strongly on the dispersion quality and interfacial interaction between the graphene and polymer matrix, the selection of graphene/polymer nanocomposites preparation methods needs to be taken into serious consideration. In this work, two preparation methods, namely in situ polymerization and solution blending are proposed to synthesize the RGO/PMMA nanocomposites. The effect of these two preparation methods on the morphology and mechanical properties of the RGO/PMMA nanocomposites will be reported and discussed. Moreover, the optimum concentration of RGO/PMMA nanocomposites for achieving the highest solar energy conversion efficiency will be investigated. 2. Materials and Methods 2.1. Raw Materials Graphite nanofiber (GNF) was supplied by Platinum Sdn. Bhd, Senawang, Malaysia. Whereas Methyl methacrylate (MMA, 99%) was acquired from Merck, Selangor, Malaysia. Polymethyl methacrylate, PMMA (120,000 molecular weight) and hydrazine hydrate solution (78%–82% iodometric) were purchased from Sigma Aldrich, Subang Jaya, Malaysia. Sulfuric acid (95%), hydrogen peroxide (30%), hydrochloric acid (37%), chloroform (stabilized with 0.6%–1.0% ethanol) and azobisisobutyronitrile (AIBN) were purchased from R&M Chemicals, Semenyih, Malaysia. Potassium permanganate and sodium nitrate were obtained from Bendosen Laboratory Chemicals, Johor Bahru, Malaysia. Deionized water was used throughout oxidation, reduction and polymerization process. 2.2. Preparation of Graphene Oxide (GO) Graphene oxide (GO) was prepared following the conventional Hummer’s method [29]. 5 g of GNF and 2.5 g of sodium nitrate were added into a beaker filled with 115 mL of sulfuric acid. After the sodium nitrate dissolved, 15 g of potassium permanganate was slowly added followed by 230 mL of deionized water. The solution was then stirred for 3 h before being poured into 700 mL of deionized water and mixed with 12 mL of hydrogen peroxide. The mixture was then left overnight and filtered using Anodisc membrane. The filtrate cake was washed with 5% hydrochloric acid and then with
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deionized water repeatedly until the pH of the supernatant reached 5–7. Finally, the product was dispersed in deionized water and dried overnight in an oven at 60 ◦ C. 2.3. Preparation of Reduced Graphene Oxide (RGO) 500 mg of GO was dispersed in 500 mL of deionized water and then exfoliated in an ultrasonicator bath for 1 h. Subsequently, 5 mL of hydrazine hydrate was added and the solution was refluxed for 24 h. The solution turned from brown to black precipitate. The product was filtered, washed with deionized water and dried at 60 ◦ C in vacuum oven. 2.4. Preparation of RGO-PMMA Composite 2.4.1. In Situ Polymerization of RGO-PMMA Nanocomposite RGO was dispersed in MMA in a glass reactor and sonicated for 1 h, followed by the adding of 0.5 wt % initiator AIBN. The reaction mixture was then heated in a water bath at 70 ◦ C for 4 h and in an oven at 100 ◦ C overnight for the completion of polymerization. 2.4.2. Solution Blending of RGO-PMMA Nanocomposite RGO and PMMA were stirred separately in chloroform. Subsequently, the RGO/chloroform solution was added to the PMMA/chloroform solution and stirred for 1 h. The solution was then poured into the Teflon mould. Finally, the chloroform was evaporated in vacuum oven at 50 ◦ C. 2.5. Characterization of Nanofillers and Nanocomposites Raman spectra of GNF and GO were recorded using NT-MDT NTEGRA Spectra instrument in order to observe the structural information of the samples. The excitation wavelength was 473 nm and the laser power was set to 1.7 mW. All powder samples were directly deposited on the glass slide in the absence of solvents. The XPS data were taken using ULVAC-PHI Quantera II equipped with monochromatic Al Kα X-ray source (hν = 1486.6 eV). All spectra were collected at room temperature. The ratio of C to O atomic element composition for GNF, GO and RGO was measured by calculating the peak areas of C and O elements from the spectra. Additionally, the infrared spectra of GNF, GO, RGO and RGO-PMMA nanocomposites were recorded using Nicolet photo-spectrometer 8700 to obtain the information about functional groups in the sample. The KBr pellet was used to analyze the absorption band with a wavelength range from 4000 to 400 cm−1 with 32 scans. Tensile testing was carried out in accordance with the standard test method ASTM D882-10 by employing Tinius Olsen H10KS lightweight tensile tester with a crosshead speed of 5 mm/min. The morphologies of GNF, GO, RGO and fracture surfaces of RGO/PMMA nanocomposites at different magnifications were captured by JEOL JSM-6701F scanning electron microscope (SEM, JEOL Inc., Massachusetts, MA, USA). Each sample was examined after sputter coating with a thin layer of platinum to avoid electrostatic charging and poor image resolution. A Hitachi HT7700 transmission electron microscope (TEM) was employed to acquire the images of RGO, RGO/PMMA nanocomposites at an accelerating voltage of 100 KV. Samples were sectioned in epoxy resin embedment before being viewed using TEM. 2.6. Solar Energy Conversion Efficiency The specific heat capacity of every nanocomposite sample was measured using Mettler Toledo TOPEM differential scanning calorimeter (DSC, Mettler Toledo Inc., Greifensee, Switzerland) prior to measuring their solar energy conversion efficiencies. The solar energy conversion efficiency measurement was carried out under outdoor conditions.
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2.6.1. Specific Heat Capacity Measurement Polymers 2017, 9, 230
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The specific heat capacity of nanocomposite samples from the DSC result can be calculated by: Polymers 2017, 9, 230 4 of 16
∆Q Q
∆Q ∆t csc = Q = Qt s m ∆T s mst∆T T ms QT m ∆t c
(1)
(1)
s
(1) Tt m −1 ·K−1 ) and mass of the s where cs and ms are specific heat capacity of the nanocomposite sample (Jg t sample where cs and ms are specific heat capacity of the nanocomposite (Jg−1·K−1) and mass of the s
ms T
sample (g), respectively. ∆Q is heat energy supplied to the nanocomposite sample (J), ∆T is the change samplecs(g), ΔQheat is heat energy to the nanocomposite sample ΔT of is the the −1) and(J), where andrespectively. ms are specific capacity of supplied the nanocomposite sample (Jg−1·K mass in temperature and ∆t is the period of period time for the change in temperature ∆T (s). change in (K) temperature (K) and Δt is the of time for the change in temperature ΔT (s). sample (g), respectively. ΔQ is heat energy supplied to the nanocomposite sample (J), ΔT is the change inSolar temperature (K) and Δt is theEfficiency period of time for the change in temperature ΔT (s). 2.6.2. Outdoor Energy Conversion 2.6.2. Outdoor Solar Energy Conversion Efficiency Measurement Measurement
2.6.2.Samples Outdoor Solar Energynanocomposites Conversion Efficiency Measurement of RGO/PMMA nanocomposites were placed in in the the polystyrene polystyrene box exposed to to the Samples of RGO/PMMA were placed boxand and exposed sun as illustrated in Figure 1. sun as the illustrated in Figure 1. Samples of RGO/PMMA nanocomposites were placed in the polystyrene box and exposed to the sun as illustrated in Figure 1.
(a)
(b)
Figure 1. (a) Experiment(a)setup for solar energy conversion system testing; (b) (b) Heat transfer of a
Figure 1. (a) Experiment setup for solar energy conversion system testing; (b) Heat transfer of nanocomposites sample. Figure 1. (a) sample. Experiment setup for solar energy conversion system testing; (b) Heat transfer of a a nanocomposites nanocomposites sample.
Figure 2 illustrates the outdoor experimental setup for solar energy conversion efficiency measurement. A KiPPthe & the Zonen CMP3 Pyranometersetup was used tosolar measure the solar irradiance in FigureFigure 2 illustrates outdoor experimental setup forsolar energy conversion efficiency 2 illustrates outdoor experimental for energy conversion efficiency Watts per meter square. Temperatures of the nanocomposite samples (denoted as T s1–Ts10) were measurement. A KiPP & Zonen CMP3 Pyranometer was used totomeasure in Watts measurement. A KiPP & Zonen CMP3 Pyranometer was used measurethe thesolar solar irradiance irradiance in recorded using thermocouples connectedoftothe Graphtec data logger GL820,(denoted before being processed by Watts per meter square. Temperatures nanocomposite samples as T s1 –T s10 ) were per meter square. Temperatures of the nanocomposite samples (denoted as Ts1 –Ts10 ) were recorded using a computer. recorded using thermocouples connected to Graphtec data logger GL820, before being processed by
thermocouples connected to Graphtec data logger GL820, before being processed by a computer. a computer.
Figure 2. Outdoor experiment setup for solar energy conversion efficiency measurement. Figure 2. Outdoor experiment setupfor forsolar solar energy conversion efficiency measurement. Figure 2. Outdoor setup energy conversion efficiency Heat transfer for aexperiment nanocomposite is shown in Figure 1b. Solar energy measurement. (Esolar) is generally absorbed at the top surface of the sample. A portion of the solar energy is converted into heat energy Heat transfer for a nanocomposite is shown in Figure 1b. Solar energy (Esolar ) is generally
Heat transfer fortop a nanocomposite is shown in Figure Solar energy (Esolar ) is generally absorbed absorbed at the surface of the sample. A portion of the1b. solar energy is converted into heat energy at the top surface of the sample. A portion of the solar energy is converted into heat energy (Eheat ), which substantially increases the temperature of the sample (Ts ). On the other hand, sunlight reflected
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Polymers 2017, 9, substantially 230 (Eheat), which
5 of 16 increases the temperature of the sample (Ts). On the other hand, sunlight reflected from the top surface and radiative heat transfer to the environment account for heat loss (Eloss) from the sample. Therefore, the rate of energy absorbed by the sample is represented by: from the top surface and radiative heat transfer to the environment account for heat loss (Eloss ) from the sample. Therefore, the rate of energy sample E heat absorbed E solar Eby m (2) lossthe s c s T sis represented by:
The total solar energy incident on = theE top − surface sample over a period of time(2) is Eheat Eloss =area ms csof ∆Tthe s solar given by: The total solar energy incident on the topt Nsurface area of the sample over a period of time is given by: E solar I solar t A dt (3)
ZttN0
Esolar = Isolar (t) · A dt (3) where Isolar is the solar irradiance measured by using the pyranometer (Wm−2) and A (π*D2/4) is the t=0 top surface area of the nanocomposite sample (m2). where Isolar isthe thesolar solarenergy irradiance measured by using(η)the pyranometer (Wm−2 ) as: and A (π*D2 /4) is the Finally, conversion efficiency can be thus determined top surface area of the nanocomposite sample (m2 ). E Finally, the solar energy conversion efficiency can%be thus determined as: heat(η) 100 (4)
Esolar
E η = heat × 100% Esolar
(4)
3. Results and Discussion 3. Results and Discussion 3.1. Characterization of the Nanofiller and Nanocomposites 3.1. Characterization of the Nanofiller and Nanocomposites Evaluations using FTIR, Raman spectroscopy and XPS were carried out to determine the Evaluations using FTIR, Ramanand spectroscopy and XPS were carried to determine the oxidation oxidation and reduction of GNF, to substantially characterize theout chemical compositions and and reduction of GNF, and to substantially characterize the chemical compositions and morphology of morphology of GNF, GO, RGO and RGO/PMMA nanocomposites. GNF, GO, RGO and RGO/PMMA nanocomposites. 3.1.1. Raman Spectroscopy 3.1.1. Raman Spectroscopy Strong G and D peaks can be seen from the Raman spectra in Figure 3 for both GO and RGO, Strong G and D peaks can be seen from the Raman spectra in Figure 3 for both GO and RGO, suggesting that they have very small crystal sizes [30]. According to the Raman spectrum of GO, the suggesting that they have very small crystal sizes [30]. According to the Raman spectrum of GO, G and D bands are at 1597 and 1353 cm−1,−respectively. The G band corresponds to the first-order 1 the G and D bands 2are at 1597 and 1353 cm , respectively. The G band corresponds to the first-order scattering of the E2g mode, whereas the prominent D peak indicates the reduction in size of the scattering of the E g mode, whereas the prominent D peak indicates the reduction in size of the in-plane sp2 domain. This is due to the extensive oxidation and the attachment of hydroxyl and in-plane sp2 domain. This is due to the extensive oxidation and the attachment of hydroxyl and epoxide groups on the carbon basal plane. On the other hand, the Raman spectrum of RGO indicates epoxide groups on the−1carbon basal plane. On the other hand, the Raman spectrum of RGO indicates the G band at 1603 cm and D band at 1363 cm−1. Since the D band of RGO is more intense than the G the G band at 1603 cm−1 and D band at 1363 cm−1 . Since the D band of RGO is more intense than the2 band, it implies a reduction of the exfoliated GO results in a decrement of the average size of sp G band, it implies a reduction of the exfoliated GO results in a decrement of the average size of sp2 domain [31]. domain [31].
Figure 3. Raman spectra of GO and RGO. Figure 3. Raman spectra of GO and RGO.
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3.1.2. 3.1.2. X-ray X-ray Photoelectron Photoelectron Spectroscopy Spectroscopy (XPS) (XPS) Figure Figure 44 shows shows the the core-level core-level shift shift (Cls) (Cls) XPS XPS spectra spectra of of GO GO and and RGO, respectively. respectively. ItIt is is apparent apparent from the GNF GNF was wasoxidized oxidizedtotoGO. GO.The TheCls ClsXPS XPS spectrum represented from Figure 4a that the spectrum of of GOGO areare represented by by characteristic peaks correspondingtotocarbon carbonatom atom with with different different functional groups: thethe characteristic peaks corresponding groups: the the non-oxygenated C (C–C, (C–C,284.80 284.80eV), eV),ether etherC C (C–O, 286.93 eV), carbonyl C (C=O, 288.55 eV) non-oxygenated ring C (C–O, 286.93 eV), carbonyl C (C=O, 288.55 eV) and and carboxylate C (COOH, 291.01 eV).Cls The Cls spectrum ofin RGO in Figure 4b exhibits the similar carboxylate C (COOH, 291.01 eV). The spectrum of RGO Figure 4b exhibits the similar oxygen oxygen functionalities, however, their peak intensities muchthan smaller than those in GO, owing possibly functionalities, however, their peak intensities are muchare smaller those in GO, possibly to owing to the reduced functional Besides, an additional at 285.79 corresponding the reduced oxygen oxygen functional groups.groups. Besides, an additional peakpeak at 285.79 eV eV corresponding to to nitrogen bound carbonatom atomwas wasnoticed noticed[32]. [32].This Thisindicates indicatesthat thatthe the reduction reduction of of GO to RGO nitrogen bound totocarbon RGO using using hydrazine hydrazine hydrate hydrate will will incorporate incorporatenitrogen nitrogento tothe thecarbon carbonatom atom[33]. [33].
(a)
(b) Figure 4. 4. The The C1s C1s XPS XPS spectrum spectrum of of (a) (a) GO; GO; (b) (b) RGO. RGO. Figure
3.1.3. Fourier Fourier Transform TransformInfrared InfraredSpectroscopy Spectroscopy(FTIR) (FTIR) 3.1.3. Figure 55 depicts depicts the the FTIR FTIR spectra spectra of of GNF, GNF, GO GO and and RGO. RGO.The Thepeak peakofofGNF GNFatat1565 1565cm cm −−1 1 Figure corresponds to the C=C aromatic ring chain stretching. The spectrum of GO represents the corresponds to the C=C aromatic ring chain stretching. The spectrum of GO represents the characteristic characteristic peaks corresponding to the oxygen functionalities group such as C=O peaks corresponding to the oxygen functionalities group such as C=O stretching, C–Ostretching, stretching C–O and −1 stretching and C–O bending vibration at 1723, 1256 and 1032 cm , respectively [34]. The peak at 3402 − 1 C–O bending vibration at 1723, 1256 and 1032 cm , respectively [34]. The peak at 3402 and 1386 cm−1 −1 and 1386 cm−1 to hydroxyl groups (OH), whereby the range 2500 to 3500 cm −1 confirms corresponds tocorresponds hydroxyl groups (OH), whereby the broad range ofbroad 2500 to 3500of cm the confirms of thecarboxylic presence group of carboxylic group (COOH). the cm peak 1623 cm−1the represents the −1 at presence (COOH). Moreover, theMoreover, peak at 1623 represents unoxidized unoxidized graphitic domain The GOthat spectra show that the using oxidation of GNF using graphitic domain (C=C). The GO(C=C). spectra show the oxidation of GNF Hummer’s method Hummer’s method was successful through the incorporation of the oxygen functional group. The was successful through the incorporation of the oxygen functional group. The reduction of GO was reduction of GO was confirmed by the FTIR spectrum of RGO in that the peak intensity of all oxygen confirmed by the FTIR spectrum of RGO in that the peak intensity of all oxygen functionality groups −1 (represents OH) in functionality groups was reduced Notecm that peak at 3435 cmthe −1 the was reduced significantly. Note thatsignificantly. the peak at 3435 (represents OH) in FTIR spectrum of the FTIR spectrum of RGO indicates theincomplete. reduction of GO was incomplete. RGO indicates the reduction of GO was The FTIR FTIR spectra spectra of of RGO, RGO, PMMA PMMA and and the the nanocomposites nanocomposites prepared prepared by by in in situ situ polymerization polymerization The and solution blending are shown in Figures 6 and 7, respectively. As can be seen, 2.0% RGO PMMA PMMA and solution blending are shown in Figures 6 and 7, respectively. As can be seen, 2.0% RGO −1 compared to the neat PMMA. nanocomposites have a broader and more intense peak at 3448 cm − 1 nanocomposites have a broader and more intense peak at 3448 cm compared to the neat PMMA. This may maybebedue due presence of hydroxyl the hydroxyl the In RGO. In addition, interaction of This to to thethe presence of the groupgroup in thein RGO. addition, interaction of PMMA PMMA and RGO may not involve chemical reaction, as no new functional group was obviously and RGO may not involve chemical reaction, as no new functional group was obviously noticed under noticed under these preparation methods. these preparation methods.
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Figure 5. FTIR spectra of GNF, GO and RGO. Figure 5. 5. FTIR FTIRspectra spectra of of GNF, GNF, GO GO and RGO. Figure
Figure 6. FTIR spectra of RGO, PMMA and nanocomposites prepared by in situ polymerization. Figure 6. FTIR spectra of RGO, PMMA and nanocomposites prepared by in situ polymerization.
Figure 6. FTIR spectra of RGO, PMMA and nanocomposites prepared by in situ polymerization.
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Figure Figure 7. 7. FTIR FTIR spectra spectra of of RGO, RGO, PMMA PMMA and and nanocomposites nanocomposites prepared prepared by by solution solutionblending. blending.
3.2. 3.2. Mechanical Mechanical Properties Properties The The effect effect of of the the preparation preparation method method and and RGO RGO loading loading on on the the tensile tensile properties properties of of the the nanocomposites are summarized summarized in inTable Table1.1.It It apparent that neat PMMA prepared viasitu in nanocomposites are is is apparent that thethe neat PMMA prepared via in situ polymerization exhibits relatively better performance than solution blending. In particular, for polymerization exhibits relatively better performance than solution blending. In particular, for in in situ situ nanocomposites, nanocomposites, the the Young’s Young’s modulus modulus achieves achieves its its peak peak at at 0.5 0.5 wt wt % % RGO RGO loading, loading, but but is is at at its its minimal minimal value value at at 2.0 2.0 wt wt % % loading loading within within the the design design window. window. Likewise, Likewise, reduction reduction of of tensile tensile strength strength can be seen. This may be caused by formation of an intercalated structure of RGO in the polymer can be seen. This may be caused by formation of an intercalated structure of RGO in the polymer matrix. the RGO sheets, only expanding the the distance between the matrix. The Themonomers monomerspolymerize polymerizebetween between the RGO sheets, only expanding distance between sheets without fully destroying the ordered structure.structure. In other words, an words, inhomogeneous dispersion the sheets without fully destroying the ordered In other an inhomogeneous of RGO will be achieved only if few monomers manage to get in between the sheets. This will reduce dispersion of RGO will be achieved only if few monomers manage to get in between the sheets. This the interaction the RGO and PMMA, and subsequently the stress willsurface reducearea thefor surface area between for interaction between the RGO and PMMA, decrease and subsequently transfer decreaseefficiency. the stress transfer efficiency. With With regard regard to to solution solution blended blended PMMA PMMA nanocomposites, nanocomposites, on on the the other other hand, hand, the the addition addition of of RGO RGO up up to to 2.0 2.0 wt wt % % leads leads to to improvement improvement of of Young’s Young’s modulus, modulus, up up to to 46.79% 46.79% compared compared to to PMMA PMMA without without RGO. RGO. The The tensile tensile strength strength with with 0.5 0.5 wt wt % % loading loading of of RGO RGO leads leads to to slight slight improvement improvement of of 3.03%. However, addition beyond 0.5 wt % decreases the tensile strength, which may 3.03%. However, addition beyond 0.5 wt % decreases the tensile strength, which may be be attributed attributed to RGO agglomeration agglomerationeffect effectand andpoor poor stress transfer [35]. elongation at break decreases to the the RGO stress transfer [35]. TheThe elongation at break decreases with with increasing RGO loading for nanocomposites prepared by both solution blending and in increasing RGO loading for nanocomposites prepared by both solution blending and in situ situ polymerization TheThe decrease in elongation at break at maybreak be attributed demobilization polymerizationmethods. methods. decrease in elongation may beto the attributed to the of polymer chains on the surface RGO, which reduces flexibility of the the flexibility chain when demobilization of polymer chainsofon the surface of RGO,the which reduces of the the RGO chain loading increases. The decrease in elongation at break is more prominent for nanocomposites prepared when the RGO loading increases. The decrease in elongation at break is more prominent for by in situ polymerization to situ inhomogeneous dispersion of RGO. nanocomposites prepareddue by in polymerization due to inhomogeneous dispersion of RGO. To better illustrate the mechanical behavior, the stress-strain curves of the To better illustrate the mechanical behavior, the stress-strain curves of the nanocomposites nanocomposites are are shown Figure 8. 8. InInthe the figure, a ductile behavior be seen for neat the neat PMMA prepared shown in in Figure figure, a ductile behavior cancan be seen for the PMMA prepared by in by situ polymerization method. However, all other samples displayed brittle fracture behavior. situinpolymerization method. However, all other samples displayed brittle fracture behavior. For For nanocomposites prepared by the in situ polymerization method, the stress at break decreased with nanocomposites prepared by the in situ polymerization method, the stress at break with increasing forfor those prepared by solution blending method, the stress at break increasingRGO RGOloading. loading.Whereas, Whereas, those prepared by solution blending method, the stress at was increased by adding 0.5% RGO, decreased with further RGO loadings. break was increased by adding 0.5%but RGO, but decreased with further RGO loadings.
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Figure 8. Stress-strain curves for various nanocomposites prepared by in situ polymerization and Figure 8. blending Stress-strain curves for various nanocomposites prepared by in situ polymerization and solution method. Figure 8. Stress-strain curves for various nanocomposites prepared by in situ polymerization and solution blending method. solution blending method. Table 1. Mechanical properties for neat PMMA and its nanocomposites. Table 1. Mechanical properties for neat PMMA and its nanocomposites. Young’s modulus Tensile strength Elongation at break Tableloading 1. Mechanical Method RGO (wt %)properties for neat PMMA and its nanocomposites. (MPa) (MPa) (%) RGO loading Young’s modulus Tensile strength Elongation at Method Young’s modulus Tensile strength Elongation at break In situ 0.0 362.28 ± 9.79 47.73 ± 1.75 29.96 (MPa) (MPa) break (%) ± 2.77 Method RGO loading (wt (wt%) %) (MPa) (MPa) (%) 0.5 428.51 ± 32.32 44.10 ± 3.02 13.50 ± 1.13 In situ 0.0 362.28 ± 9.79 47.73 ± 1.75 29.96 ± 2.77 In situ 0.0 362.28 9.79 47.73 1.75 29.96 2.77 2.0 288.72 ±±± 48.96 29.68 1.25 13.95 0.5 428.51 32.32 44.10 ±±±3.02 13.50 ± 1.13 ±± 1.76 0.5 428.51 32.32 44.10 3.02 2.0 288.72± ± 48.96 29.68 ± ±1.25 13.95 ±13.50 1.76 ± 1.13 Solution 0.0 330.47 ± 26.00 35.89 ± 1.88 15.10 ±± 2.61 2.0 288.72 48.96 29.68 1.25 Blending Solution Blending 0.0 330.47± ± 26.00 35.89 ± ±1.88 15.10 ±13.95 2.61 1.76 Solution 0.5 463.02 47.66 36.64 10.02 ± 1.10 ± 1.10 0.5 463.02 ±± 47.66 36.64±±1.18 1.18 10.02 0.0 330.47 ± 26.00 35.89 ± 1.88 15.10 ± 2.61 2.0 470.72 ± 33.34 25.51 ± 6.01 6.94 ± 1.13 Blending 2.0 470.72 ± 33.34 25.51 ± 6.01 6.94 ± 1.13 0.5 463.02 ± 47.66 36.64 ± 1.18 10.02 ± 1.10 470.72 ± 33.34 25.51 ± 6.01 6.94 ± 1.13 3.3. 3.3. Morphology Morphology Properties Properties 2.0
3.3.1. Scanning Microscope 3.3. Morphology Properties 3.3.1. Scanning Electron Electron Microscope(SEM) (SEM) Figure 99 displays the FESEM FESEM images images of of GNF, GNF, GO GO and and RGO RGO morphology morphology at at 20,000 20,000 times times 3.3.1.Figure Scanningdisplays Electron the Microscope (SEM) magnification. Both GNF and GO possessed hair-like fiber structures; however, the GO fiber structure magnification. Both GNF and GO possessed hair-like fiber structures; however, the GO fiber Figure displays the and FESEM images of GNF, GOwhen and compared RGO morphology atThis 20,000 times was torn into pieces the surface was rougher, with GNF.with could be structure was9smaller torn into smaller pieces and the surface was rougher, when compared GNF. This magnification. Both GNF and GO possessed hair-like fiber structures; however, the GO fiber attributed to the oxygen-containing groups present onpresent the surface of surface the graphitic could be attributed to the oxygen-containing groups on the of the layer. graphitic layer. structure was torn into smaller pieces and the surface was rougher, when compared with GNF. This could be attributed to the oxygen-containing groups present on the surface of the graphitic layer.
(a) (a)
(b) Figure9. 9. SEM SEM images images of of (a) (a) GNF; GNF;(b) (b)GO; GO;(c) (c)RGO. RGO. (b) Figure
(c) (c)
Figure 9. SEM images of (a) GNF; (b) GO; (c) RGO. The cross-section morphology of the fractured nanocomposites was recorded by SEM at the The cross-section morphology of the fractured nanocomposites was recorded by SEM at the magnification of ×500. The tensile fracture surfaces of the neat PMMA prepared by both methods are magnification of ×500. morphology The tensile fracture surfaces ofnanocomposites the neat PMMA prepared by both methods Thefrom cross-section the fractured recorded by SEM at distinct each other, as shown inofFigure 10. Neat PMMA throughwas in situ polymerization hasthe a are distinct from each other, as shown in Figure 10. Neat PMMA through in situ polymerization has magnification of ×500. The tensile fracture surfaces of the neat PMMA prepared by both methods are relatively smooth surface, whereas those prepared through solution blending have prominent crack adistinct relatively smooth prepared through solution blending prominent crack from each surface, other, aswhereas shown those in Figure 10. Neat PMMA through in situhave polymerization has a growth on a major plane. growth on smooth a major plane. relatively surface, whereas those prepared through solution blending have prominent crack
growth on a major plane.
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Figure 10. Tensile fracture surfaces of neat PMMA prepared by different methods. (a) In situ
Figure 10. Tensile fracture methods. (a) In situ (a) surfaces of neat PMMA prepared by different (b) polymerization; (b) solution blending. (a)blending. (b) polymerization; (b) solution
Figure 10. Tensile fracture surfaces of neat PMMA prepared by different methods. (a) In situ Tensile10. strength PMMA prepared by inPMMA situ polymerization was decreased adding Figure Tensile fracture surfaces of neat prepared by different methods.by(a) In situRGO. polymerization; (b)for solution blending. Tensile strength for PMMA prepared by in situ polymerization was by adding According to Figure 11, voids and RGO agglomerates can be observed on thedecreased tensile fracture surfaceRGO. polymerization; (b) solution blending.
of nanocomposites. of agglomerate area ofdecreased RGO for by interaction with of Tensile strength for PMMA prepared by in reduce situ polymerization was adding RGO. According to Figure 11, Formations voids and RGO agglomerates canthe besurface observed on the tensile fracture surface Tensile strength for PMMA prepared by in situ polymerization was decreased by adding RGO. the polymer matrix and lead to poor adhesion between filer and matrix [36]. Voids can be attributed According to Figure 11, voids and RGO agglomerates can be observed on the tensile fracture surface nanocomposites. Formations of agglomerate reduce the surface area of RGO for interaction with the According Figure 11, of voids and agglomerates be observed on the tensile fracture surface to the easy to detachment these agglomerates from thecan matrix [37]. area These agglomerates become of nanocomposites. of RGO agglomerate reduce the RGO for interaction withto the polymer matrix and leadFormations to poor adhesion between filer andsurface matrix [36].ofVoids can be also attributed of nanocomposites. Formations of nanocomposite agglomerate reduce the surface area of RGO interaction with stress concentration points system. 12 shows the for tensile strength for the polymer matrix and leadintothe poor adhesion between filerFigure and matrix [36]. Voids can be attributed easy detachment of these agglomerates from the matrix [37]. These agglomerates also become stress the polymer matrix andof lead to poor adhesion between filer and matrix [36]. Voids can be attributed solution blended PMMA nanocomposites. RGO agglomerates and voids can be clearly seen when to the easy detachment these agglomerates from the matrix [37]. These agglomerates also become concentration points in theofnanocomposite system. Figure 12 [37]. shows theagglomerates tensile strength for solution to easy detachment agglomerates from the matrix These become thethe RGO loading is increased 0.5 to 2.0 wt %. system. stress concentration pointsthese infrom the nanocomposite Figure 12 shows the tensile also strength for blended PMMA nanocomposites. RGO agglomerates and voids can be clearly seen when the stress concentration pointsnanocomposites. in the nanocomposite system. Figure shows the forRGO solution blended PMMA RGO agglomerates and12voids can betensile clearlystrength seen when loading increased 0.5nanocomposites. to 2.0 wt0.5%.to 2.0RGO solution blended PMMA the is RGO loadingfrom is increased from wt %.agglomerates and voids can be clearly seen when the RGO loading is increased from 0.5 to 2.0 wt %.
(a)
(b)
Figure 11. Tensile fracture surfaces of nanocomposites prepared by in situ polymerization with (a) (a) (b) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading. (a) (b) Figure 11. Tensile fracture surfaces of nanocomposites prepared by in situ polymerization with (a) Figure 11. 11. Tensile fracture surfaces of nanocomposites prepared by in situ polymerization with Figure Tensile fracture surfaces of nanocomposites prepared by in situ polymerization with (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading. (a) 0.50.5wtwt%%RGO loading; (b) 2.0 wt % RGO loading. RGO loading; (b) 2.0 wt % RGO loading.
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(b)
Figure 12. Tensile fracture surfaces of nanocomposites prepared by solution blending with different (a) (b) RGO loading. (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading. (a) (b) Figure 12. Tensile fracture surfaces of nanocomposites prepared by solution blending with different Figure 12. Tensile0.5 fracture surfaces of nanocomposites prepared by solution blending with different RGO wt surfaces % RGO loading; (b) 2.0 wt % RGO loading. Figure 12. loading. Tensile (a) fracture of nanocomposites prepared by solution blending with different RGO loading. (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading.
RGO loading. (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading.
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3.3.2. Transmission Electron 3.3.2. Transmission Electron Microscope Microscope (TEM) (TEM) 3.3.2. Transmission Electron Microscope (TEM) RGO RGO is is aa smooth smooth and and thin thin layer layer of of sheets sheets stacked stacked together together as as shown shown in in Figure Figure 13. 13. The The number number of of RGO is a smooth and thin layer of sheets stacked together as shown in Figure 13. The number of layers ranges from 2 to 4 by visual inspection, and the average length is found to be 738.5 ± 141.1 nm. layers ranges from 2 to 4 by visual inspection, and the average length is found to be 738.5 ± 141.1 nm. layers 2 to 4 by visual inspection, and average length ishave found to be 738.5 141.1 nm. In present study, nanocomposites prepared by solution blending aa better RGO In the the ranges presentfrom study, nanocomposites prepared bythe solution blending have better RGO±dispersion dispersion In the present study, nanocomposites prepared by solution blending have a better RGO dispersion than This can can be than those those prepared prepared by by in in situ situ polymerization. polymerization. This be confirmed confirmed from from the the cross-section cross-section TEM TEM than those prepared by in situ polymerization. This can be confirmed from the cross-section TEM images of nanocomposites prepared by in situ polymerization and solution blending methods, shown images of nanocomposites prepared by in situ polymerization and solution blending methods, images of 14 nanocomposites prepared by in Apparently, situ polymerization and solution blending methods, in Figures and 15,14respectively. Apparently, the nanocomposites prepared by inprepared situ polymerization shown in Figures and 15, respectively. the nanocomposites by in situ shown in Figures 14 and 15, respectively. Apparently, the nanocomposites prepared by in have more aggregate numbers of multi-layer RGO, which is densely agglomerated, as indicated bysitu the polymerization have more aggregate numbers of multi-layer RGO, which is densely agglomerated, polymerization have more aggregate numbers of multi-layer RGO, which is densely agglomerated, darker image of multilayer RGO. as indicated by the darker image of multilayer RGO. as indicated by the darker image of multilayer RGO.
Figure 13. Cross section TEM image of RGO. Figure 13. 13. Cross Cross section section TEM TEM image image of of RGO. RGO. Figure
(a) (b) (a) (b) Figure 14. Cross section TEM images of nanocomposites prepared by in situ polymerization with Figure 14.RGO Cross section images of nanocomposites by situ polymerization polymerization with with Figure 14. Cross section(a)TEM TEM images of loading; nanocomposites prepared by in in situ different loading. 0.5 wt % RGO (b) 2.0 wtprepared % RGO loading. different RGO loading. (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading. different RGO loading. (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading.
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(b)(b)
Figure15. 15.Cross Cross section section TEM TEM images images of prepared bybysolution blending with Figure of nanocomposites nanocomposites prepared solution with Figure 15. Cross section TEM images of nanocomposites prepared by solution blendingblending with different different RGO loading. (a) 0.5 wt % RGO loading; (b) 2.0 wt % RGO loading. different RGO(a) loading. (a)RGO 0.5 wt % RGO(b) loading; 2.0 wt % RGO loading. RGO loading. 0.5 wt % loading; 2.0 wt (b) % RGO loading.
3.4. Solar Energy Conversion Efficiency 3.4. Solar Solar Energy EnergyConversion ConversionEfficiency Efficiency 3.4. 3.4.1. Specific Heat Capacity 3.4.1. Specific SpecificHeat HeatCapacity Capacity 3.4.1. The calculation results of specific heat capacities from the DSC are displayed in Figure 16. The calculation calculation results ofnanocomposites specific heat heat capacities capacities from the DSC DSC are displayed displayed in Figure Figure 16. The specific the are in Generally, PMMA results and itsof prepared from by solution blending have higher heat16. Generally, PMMA and its nanocomposites prepared by solution blending have higher heat Generally, PMMA and its nanocomposites prepared by solution blending have higher heat capacities capacities than those prepared using the in situ polymerization method. capacities thoseusing prepared the in situ polymerization than those than prepared the inusing situ polymerization method. method.
(a)
(b)
Figure 16. Specific heat prepared by (a)capacity of the neat PMMA and RGO/PMMA nanocomposites (b) (a) in situ polymerization and (b) solution blending. Figure16. 16.Specific Specificheat heatcapacity capacityofofthe theneat neatPMMA PMMAand andRGO/PMMA RGO/PMMA nanocomposites nanocomposites prepared prepared by by Figure (a) in situ polymerization and (b) solution blending. 3.4.2. Solar Conversion Efficiency (a) in situ Energy polymerization and (b) solution blending.
The nanocomposites wereEfficiency thermally insulated at all surfaces except the top surface during the 3.4.2. Solar Energy Conversion 3.4.2. Solar experiment. Energy Conversion Efficiency outdoor The thermal energies absorbed by the nanocomposites were calculated from The nanocomposites were thermally insulated Heat at all surfaces except the top surface during the theThe rise in temperature of the nanocomposites. loss is assumed besurface negligible. Thethe nanocomposites were thermally insulated at all surfaces except thetotop during outdoor experiment. The thermal energies absorbed bytrend the nanocomposites were calculated from temperature of the nanocomposites showsabsorbed an increasing when taking the measurement under outdoor experiment. The thermal energies by the nanocomposites were calculated from the theconstant rise in solar temperature of the nanocomposites. Heat loss is assumed to be negligible. The radiation. Figure 17 shows the daily temperature data which fulfilled the rise in temperature of the nanocomposites. Heat loss is assumed to be negligible. The temperature of temperature of the nanocomposites shows an increasing trend when taking the measurement under conditions follows: themeasurement nanocomposites showsasan increasing trend when taking the measurement under constant solar constant solar radiation. Figure 17 shows the daily temperature data which fulfilled the radiation. shows the ranged daily temperature (1) TheFigure starting17temperature from 30 to 40data °C; which fulfilled the measurement conditions measurement conditions as follows: The solar irradiance ranged from 500 to 1000 W/m2; as (2) follows: Thestarting temperatures of all nanocomposites (1)(3) The temperature ranged from 30increased to 40 ◦°C;for more than 20 s; (1) (4) The starting temperature rangedwere from 30 to 40 1% C; 2 The changes in solarranged irradiance than the period of time. (2) The solar irradiance from 500 less to 1000 W/mduring ; 2; (2) The solar irradiance ranged from 500 to 1000 W/m (3) The temperatures of all nanocomposites increased for more than 20 s; (3) of all nanocomposites for more than 20 s;of time. (4) The The temperatures changes in solar irradiance were lessincreased than 1% during the period (4) The changes in solar irradiance were less than 1% during the period of time.
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Figure 17. Sets of results that fulfill the required conditions. Figure 17. Sets of results that fulfill the required conditions.
Figureenergy 17. Sets of results that fulfill the required The results of solar conversion efficiencies of 10conditions. neat PMMA and PMMA/RGO nanocomposites prepared by the two different methods are compared in Figure 18. The efficiencies The results of solar energy conversion efficiencies of 10 neat PMMA and PMMA/RGO resultsare of the solaraverage energyefficiencies conversionofefficiencies 10 neatfrom PMMA anddays PMMA/RGO in The the graph 25 results of obtained several of outdoor nanocomposites prepared by the two different methods are compared in Figure 18. The efficiencies nanocomposites prepared by the two different methods are compared in Figure 18. The efficiencies experiment that met all required conditions. The energy conversion efficiency of the PMMA andinits in the graph are the average efficiencies of 25 results obtained from several days of outdoor the graph areprepared the average efficiencies 25 results obtained several of outdoor composite by the solutionof blending method wasfrom found to be days relatively higher.experiment Besides, the experiment that met all required conditions. The energy conversion efficiency of the PMMA and its that met all required conditions. The energy conversion efficiency of the PMMA and its composite maximum efficiency of about 25% can be achieved when 1% RGO was added to the PMMA. This composite prepared by the solution blending method was found to be relatively higher. Besides, the prepared thetosolution method of was found to bewithout relatively higher. Besides, the maximum may be by due the factblending that the surface neat PMMA RGO is white in color. By adding maximum efficiency of about 25% can be achieved when 1% RGO was added to the PMMA. This efficiency of surface about 25% canblack, be achieved when 1% RGO was added and to the PMMA. This may dueheat. to RGO, the turns resulting in higher absorptivity lower emissivity of be solar may be due to the fact that the surface of neat PMMA without RGO is white in color. By adding the fact that the surface of neat PMMA without RGO is white in color. By adding RGO, the surface turns Consequently, the energy conversion efficiency is enhanced. Nevertheless, subsequently increasing RGO, the surface turns black, resulting in higher absorptivity and lower emissivity of solar heat. black, resulting indid higher lower emissivity of solar heat. Consequently, the energy the RGO to 2% not absorptivity improve the and efficiency in the present study. This may be because both the Consequently, the energy conversion efficiency is enhanced. Nevertheless, subsequently increasing conversion efficiency is enhanced. subsequently increasing the RGOlarge to 2% did notof absorptivity and emissivity were Nevertheless, high in this case; hence, the surface emitted amounts the RGO to 2% did not improve the efficiency in the present study. This may be because both the improve efficiency theform present Thisheat mayloss be because both the absorptivity emissivity thermalthe radiation ininthe of study. radiative to the environment, and thisand decreased the absorptivity and emissivity were high in this case; hence, the surface emitted large amounts of were high in this case; hence, the surface emitted large amounts of thermal radiation in the form of efficiency. thermal radiation in the form of radiative heat loss to the environment, and this decreased the radiative heat loss to the environment, and this decreased the efficiency. efficiency.
Figure Solar energy conversion efficiency neat PMMA and nanocomposites. Figure 18.18. Solar energy conversion efficiency ofof thethe neat PMMA and itsits nanocomposites. Figure 18. Solar energy conversion efficiency of the neat PMMA and its nanocomposites.
order investigate the effect solar irradiance, the efficiency increment nanocomposites InIn order toto investigate the effect ofof solar irradiance, the efficiency increment ofof nanocomposites mixed with 1% RGO is illustrated in Figure 19. The optimum temperature for the nanocomposites is mixed with 1% RGO is illustrated in of Figure The optimum temperature for theofnanocomposites In order to investigate the effect solar19. irradiance, the efficiency increment nanocompositesis found a range of 30–45 In addition, the nanocomposites may absorb the heat without ◦ C. In °C. found as as a range of 30–45 addition, the19. nanocomposites may absorb the without significant mixed with 1% RGO is illustrated in Figure The optimum temperature for heat the nanocomposites is 2. As a result, the nanocomposites significant heat loss when the solar irradiance is 500–1000 W/m 2 heat loss when the solar irradiance is 500–1000 W/m . As a result, the nanocomposites prepared by found as a range of 30–45 °C. In addition, the nanocomposites may absorb the heat without prepared by the solution blending method exhibit greater conversion capability than those the solution blending method exhibit greater conversion thosethe prepared byprepared in situ significant heat loss when the solar irradiance is 500–1000capability W/m2. Asthan a result, nanocomposites by in situ polymerization within the measurement domain. polymerization within the measurement prepared by the solution blending methoddomain. exhibit greater conversion capability than those prepared by in situ polymerization within the measurement domain.
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Figure 19. Increment of efficiencies with 1% RGO prepared by (a) in situ polymerization and (b) Figure 19. Increment of efficiencies with 1% RGO prepared by (a) in situ polymerization and (b) solution solution blending. blending.
It is worth noting that the nanocomposites prepared by in situ polymerization perform better It is worth noting that the nanocomposites2 prepared by in situ polymerization perform better under a low solar irradiance of 500–750 W/m , whereas those by the solution blending method under a low solar irradiance of 500–750 W/m2 , whereas those by the solution blending method require require a higher solar irradiance of between 750 and 21000 W/m2. Furthermore, when the solar a higher solar irradiance of between 750 and 1000 W/m . Furthermore, when2 the solar irradiance is irradiance is very low during cloudy periods, ranging from 0 to 500 W/m , the efficiency of the very low during cloudy periods, ranging from 0 to 500 W/m2 , the efficiency of the nanocomposites is nanocomposites is lower than that of the neat PMMA without RGO loading. This is because the lower thancontribution that of the neat without loading. This is because negative contribution of negative of PMMA the heat loss RGO to the environment throughthethe top surface of the the heat loss to the environment through the top surface of the nanocomposites is more significant in nanocomposites is more significant in this case due to the low solar irradiance. The heat loss, as the this case due to the low solar irradiance. Thedarker heat loss, as theofdominant effect, would be increased by dominant effect, would be increased by the surface the nanocomposites with RGO owing thethe darker surface of theTherefore, nanocomposites with RGO owing to the higher emissivity. Therefore, the to higher emissivity. the efficiency decreased. efficiency decreased. 4. Conclusions 4. Conclusions The results from spectroscopic studies using FTIR, Raman spectroscopy and XPS confirmed The results from spectroscopic studies using FTIR, Raman spectroscopy and XPS confirmed that that the GO was successfully reduced to RGO. Ductile behavior can be seen in the stress-strain the GO was successfully reduced to RGO. Ductile behavior can be seen in the stress-strain curves for curves for the neat PMMA prepared by the in situ polymerization method. However, all other the neat PMMA prepared by the in situ polymerization method. However, all other samples displayed samples displayed brittle fracture behavior. Better dispersion of RGO in the PMMA matrix of the brittle fracture behavior. Better dispersion of RGO in the PMMA matrix of the solution blended solution blended nanocomposites can be confirmed from the Scanning Electron Microscope (SEM) nanocomposites can be confirmed from the Scanning Electron Microscope (SEM) and Transmission and Transmission Electron Microscope (TEM) results. The optimum concentration of RGO/PMMA Electron Microscope (TEM) results. The optimum concentration of RGO/PMMA nanocomposites is nanocomposites is found as 1.0 wt % that yields the maximal solar energy conversion efficiency of found as 1.0 wt % that yields the maximal solar energy conversion efficiency of about 25%. about 25%. Acknowledgments: The authors would like to acknowledge the Ministry of Education Malaysia Acknowledgments: The authors for would likesupport to acknowledge the this Ministry [FRGS/1/2014/TK04/UTAR/02/1] financial for carrying out research.of Education Malaysia [FRGS/1/2014/TK04/UTAR/02/1] for financial support for carrying out this research. Author Contributions: Shin Yiing Kee and Koon Chun Lai carried out the research work and wrote the paper. YamunaContributions: Munusamy andShin KokYiing SengKee Ongand supervised the research andout participated in the interpretation of results Author Koon Chun Lai carried the research work and wrote the paper. and the correction of the final draft. Yamuna Munusamy and Kok Seng Ong supervised the research and participated in the interpretation of results Conflicts of Interest: Thefinal authors declare no conflict of interest. and the correction of the draft.
Conflicts of Interest: The authors declare no conflict of interest.
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